1. Field Of The Invention
The present invention relates to fluid valves. More specifically, the present invention is directed to rotary spherical plug valves.
2. Description Of The Prior Art
Rotary valves using spherical plugs mounted on the end of support arm are well-known in the art, e.g., U.S. Pat. Nos. 4,118,008; 4,074,889; 4,073,473 and German Pat. Nos. 23 55 773 and 16 75 579 German Utility Model No. 75 06 291. In these prior art rotary valves using a spherical plug, a tight sealing of the valve is achieved either by a deformation or adjustment of the plug support arm or by a deformation of the seat ring or the valve which may be elastically resilient or movable or by a deformation of an outer part of the spherical plug cap forming the closure member which is chosen such that this outer part is slightly resiliently deformable. In all these prior art rotary valves, the rigid eccentric support of the rotary plug achieves an operation that only in the closed position the plug and the seat ring engage each other. When opening the valve the spherical cap is moved away from the seat ring caused by the eccentrical location of the driving shaft with respect to the central axis of the seat ring, which causes reduced friction and improves accuracy of control. The required closing force is determined by the amount of free cross-section at the inlet, the existing differential pressure between inlet and outlet side of the valve as well as by the eccentricity of the rotary plug bearing, that means the distance between the axis of the seat ring and the central line of the driving shaft. The maximum required torque is given by the product of cross-section area, differential pressure and eccentricity. For requiring an as small as possible torque and therewith an as small as possible force to be exerted by the valve operator one tries to keep the eccentricity as small as possible. In this connection, however, there are limitations which in the first line are determined by the friction between the rotary plug and the seat ring. If the eccentricity is too small there is a very small angle between the tangent at the spherical cup in the range of the seating edge on the one side and the surface of the seat ring on the other side so that there is the danger of sticking of the plug in the valve seat. This results that, in particular, in connection with spring loaded pnuematic actuators the plug is suddenly disengaging the valve seat when the valve is opened. This makes a fine control in the range of low flow rates difficult or even impossible.